The present invention relates to a process for the production of coupling pieces from a thermoplastic, which coupling pieces possess an undercut annular groove optionally enclosing a sealing ring, in which process a sleeve, having an internal diameter which increases in steps in the direction of the free end, is initially performed on the hot end of a pipe made of a thermoplastic, the terminal part of the preformed sleeve is then postformed in order to form the undercut annular groove, and, finally, the postformed sleeve is cooled, and also relates to an improved device for carrying out this process.
Processes of this type have already been described in Dutch Patent Applications Nos. 74/08,627 and 74/08,628, filed in the name of WAVIN B. V., and in Dutch Patent Application No. 72/09,328, filed in the name of FORSHEDA IDEUTVECKLING A. B., and also in Swiss Pat. No. 490,956, filed in the name of KUNSTSTOFFWERK GEBRUDER ANGER G.m.b.H. and Co.
In these processes, the preformed sleeve obtained in the first stage is either heated completely for the purpose of postforming, or is kept entirely at the forming temperature and postformed. These processes can therefore give rise to variations in the dimensions of the coupling pieces formed, or to the appearance of internal stresses in the walls of the sleeves thus obtained.